1. Field of the Invention
The invention relates to a doctor blade, in particular for doctoring off printing ink from a surface of a printing form, comprising a flat and elongated main body having a working edge region formed in a longitudinal direction, wherein the working edge region is covered with a first coating on the basis of an electrolessly deposited nickel-phosphorus alloy, and wherein hard material particles are dispersed in the first coating. Furthermore, the invention relates to a process for producing a doctor blade.
2. Description of the Related Art
In the printing industry, doctor blades are used, in particular, for wiping excess printing ink off the surfaces of printing cylinders and printing rolls. Particularly in the case of gravure printing and flexographic printing, the quality of the doctor blade has a decisive influence on the printing result. By way of example, instances of unevenness or irregularities in the working edges of the doctor blade that are in contact with the printing cylinder lead to incomplete wiping of the printing ink off the webs of the printing cylinders. This can result in uncontrolled release of printing ink on the printing substrate.
During the wiping-off operation, the working edges of the doctor blade are pressed onto the surfaces of the printing cylinders or printing rolls and are moved in relation thereto. Particularly in the case of rotary printing presses, the working edges are therefore subjected to high mechanical stresses, which are associated with corresponding wear. In principle, doctor blades are therefore consumable items, which have to be exchanged periodically.
Doctor blades are usually formed on the basis of a steel main body with a specially shaped working edge. In order to improve the service life of the doctor blade, it is possible for the working edges of the doctor blade to additionally be provided with coatings or coverings made of metals and/or plastics. Metallic coatings often contain nickel or chromium which, if appropriate, are present in a form mixed or alloyed with other atoms and/or compounds. In this respect, the material properties of the coatings have a significant influence on the mechanical and tribological properties of the doctor blade, in particular.
WO 2003/064157 (Nihon New Chrome Co. Ltd.), which is equivalent to U.S. Pat. No. 7,152,526, describes, for example, doctor blades for the printing sector, which have a first layer of chemical nickel with hard material particles dispersed therein and a second layer with low surface energy. The second layer preferably consists of a covering made of chemical nickel with fluorine-based resin particles or of a purely organic resin.
Although doctor blades coated in this way have an improved wear resistance compared to uncoated doctor blades, the service life is still not entirely satisfactory. In addition, it has been found that uncontrolled instances of streaking can arise, in particular in the run-in phase, when such doctor blades are used, and this is likewise undesirable.
Therefore, there is still a need for an improved doctor blade which, in particular, has a longer service life and, at the same time, makes optimum wiping off possible.